The search for new therapeutic agents has been greatly aided in recent years by better understanding of the structure of proteins and other biomolecules associated with target diseases. One important class of these proteins are the sigma (a) receptors, cell surface receptors of the central nervous system (CNS) which may be related to the dysphoric, hallucinogenic and cardiac stimulant effects of opioids. From studies of the biology and function of sigma receptors, evidence has been presented that sigma receptor ligands may be useful in the treatment of psychosis and movement disorders such as dystonia and tardive dyskinesia, and motor disturbances associated with Huntington's chorea or Tourette's syndrome and in Parkinson's disease (Walker, J. M. et al, Pharmacological Reviews, 1990, 42, 355). It has been reported that the known sigma receptor ligand rimcazole clinically shows effects in the treatment of psychosis (Snyder, S. H., Largent, B. L. J. Neuropsychiatry 1989, 1, 7). The sigma binding sites have preferential affinity for the dextrorotatory isomers of certain opiate benzomorphans, such as SKF-10047, (+)-cyclazocine, and (+)-pentazocine and also for some narcoleptics such as haloperidol.
“The sigma receptor/s” as used in this application is/are well known and defined using the following citation: This binding site represents a typical protein different from opioid, NMDA, dopaminergic, and other known neurotransmitter or hormone receptor families (G. Ronsisvalle et al. Pure Appl. Chem. 73, 1499-1509 (2001)).
The sigma receptor has at least two subtypes, which may be discriminated by stereoselective isomers of these pharmacoactive drugs. SKF-10047 has nanomolar affinity for the sigma 1 (σ-1) site, and has micromolar affinity for the sigma 2 (σ-2) site. Haloperidol has similar affinities for both subtypes.
The Sigma-1 receptor is a non-opiaceous type receptor expressed in numerous adult mammal tissues (e.g. central nervous system, ovary, testicle, placenta, adrenal gland, spleen, liver, kidney, gastrointestinal tract) as well as in embryo development from its earliest stages, and is apparently involved in a large number of physiological functions. Its high affinity for various pharmaceuticals has been described, such as for SKF-10047, (+)-pentazocine, haloperidol and rimcazole, among others, known ligands with analgesic, anxiolytic, antidepressive, antiamnesic, antipsychotic and neuroprotective activity. Sigma-1 receptor is of great interest in pharmacology in view of its possible physiological role in processes related to analgesia, anxiety, addiction, amnesia, depression, schizophrenia, stress, neuroprotection, psychosis and mood disorders [Kaiser et al (1991) Neurotransmissions 7 (1): 1-5], [Walker, J. M. et al, Pharmacological Reviews, 1990, 42, 355], [Bowen W. D. (2000) Pharmaceutica Acta Helvetiae 74: 211-218] and Hayashi, T. et al, Drugs of the Future 2009, 34 (2), 137].
The Sigma-2 receptor is also expressed in numerous adult mammal tissues (e.g. nervous system, immune system, endocrine system, liver, kidney). Sigma-2 receptors can be components in a new apoptosis route that may play an important role in regulating cell proliferation or in cell development. This route seems to consist of Sigma-2 receptors joined to intracellular membranes, located in organelles storing calcium, such as the endoplasmic reticulum and mitochondria, which also have the ability to release calcium from these organelles. The calcium signals can be used in the signaling route for normal cells and/or in induction of apoptosis.
Sigma-2 receptor ligands, specially agonists, can be used as antineoplastic agents at doses inducing apoptosis or at sub-toxic doses in combination with other antineoplastic agents to revert the resistance to the drug, thereby allowing using lower doses of the antineoplastic agent and considerably reducing its adverse effects.
Additionally, Sigma-2 receptor ligands, specially antagonists, can be useful as agents for improving the weakening effects of delayed dyskinesia appearing in patients due to chronic treatment of psychosis with typical antipsychotic drugs, such as haloperidol. Sigma-2 receptors also seem to play a role in certain degenerative disorders in which blocking these receptors could be useful.
Endogenous sigma ligands are not known, although progesterone has been suggested to be one of them. Possible sigma-site-mediated drug effects include modulation of glutamate receptor function, neurotransmitter response, neuroprotection, behavior, and cognition (Quirion, R. et al. Trends Pharmacol. Sci., 1992, 13:85-86). Most studies have implied that sigma binding sites (receptors) are plasmalemmal elements of the signal transduction cascade. Drugs reported to be selective sigma ligands have been evaluated as antipsychotics (Hanner, M. et al. Proc. Natl. Acad. Sci., 1996, 93:8072-8077). The existence of sigma receptors in the CNS, immune and endocrine systems have suggested a likelihood that it may serve as link between the three systems.
In view of the potential therapeutic applications of agonists or antagonists of the sigma receptor, a great effort has been directed to find effective ligands. Different sigma receptor ligands have been reported.
For instance, WO2007098961A1 describes 4,5,6,7-tetrahydrobenzo[b]thiophene derivatives having pharmacological activity towards the sigma receptor.
Spiro[benzopyran] and spiro[benzofuran] derivatives with pharmacological activity on sigma receptors are disclosed in WO2007121976A1.
Pyrazole derivatives presenting a pyrazole group condensed with a cycloalkyl ring have been also reported as sigma ligands in WO2006021463A1.
WO2008055932A1 and WO2008055933A1 deal with 1,2,4- and 1,2,3-triazole compounds, respectively, having activity towards sigma receptors.
WO2009071657A1 also reports tricyclic triazolic compounds having activity towards sigma receptors.
U.S. Pat. No. 3,317,524A discloses substituted 1,2,3,4-tetrahydropyrazino[1,2-a]indoles and intermediates in the preparation thereof, useful as anti-inflammatory agents, as central nervous system depressants, as analgesics and as anti-convulsants.
In spite of this background, there is still a need to find further compounds that have pharmacological activity towards the sigma receptor, preferably being both effective and selective as well as having potentially good “drugability” properties, i.e. good pharmaceutical properties related to administration, distribution, metabolism and excretion.